There has been known a method of providing music having sound quality most suitable for a sound field environment in which speakers or the like of an audio system are installed, by measuring frequency characteristics of the sound field environment and adjusting the equalizer of the audio system on the basis of the measured frequency characteristics or by previously correcting output sound in accordance with the sound field.
A pseudorandom noise (PN) code and a time stretched pulse (TSP) signal are known as measurement signals for measuring frequency characteristics. Typically, a PN code is an artificial measurement signal composed of random noise. Examples of a PN code include a maximum length sequence (m-sequence) code and a Gold sequence code.
Both an m-sequence code and a Gold sequence code are generated by performing feedback using a predetermined shift register and an exclusive OR. If the length (stage number) of a shift register is n (n is a natural number), the period of the code (code length) is 2n−1. The feedback position of the shift register is obtained using a generating polynomial. If an m-sequence code is used as an output signal, the output signal is a binary sequence composed of 0s and 1s and is a signal including many direct-current components and therefore is subjected to the conversion of 0s into −1s and then outputted. As seen above, a measurement signal composed of a periodic function having a code length of 2n−1 is used to measure the frequency characteristics of a sound field.
Examples of a method for measuring the frequency characteristics of a sound field environment using such a measurement signal include a method including picking up a measurement signal outputted from a speaker using a microphone installed in the listening position and then Fourier transforming the picked-up signal to obtain the frequency characteristics (for example, see Patent Literatures 1, 2). An impulse response may be obtained by obtaining cross-correlation characteristics between an outputted measurement signal and the measurement signal picked up using a microphone while using the outputted measurement signal as a reference.